If your system uses AccountsService, you can not hide a user from the greeter screen by reconfiguring lightdm because it defers to AccountsService. That is stated very clearly in the comments in /etc/lightdm/users.conf.
What you need to do instead is to reconfigure AccountsService.
To hide a user named XXX, create a file named
1 | /var/lib/AccountsService/users/XXX |
containing two lines:
1 2 | [User] SystemAccount=true |
If the file already exists, make sure you append the SystemAccount=true line to the [User] section.